Raman amplifier, optical repeater, and raman amplification method

ABSTRACT

A Raman amplifier according to the present invention comprises a plurality of pumping means using semiconductor lasers of Fabry-Perot, DFB, or DBR type or MOPAs, and pumping lights outputted from the pumping means have different central wavelengths, and interval between the adjacent central wavelength is greater than 6 nm and smaller than 35 nm. An optical repeater according to the present invention comprises the above-mentioned Raman amplifier and adapted to compensate loss in an optical fiber transmission line by the Raman amplifier. In a Raman amplification method according to the present invention, the shorter the central wavelength of the pumping light the higher light power of said pumping light. In the Raman amplifier according to the present invention, when a certain pumping wavelength is defined as a first channel, and second to n-th channels are defined to be arranged with an interval of about 1 THz toward a longer wavelength side, the pumping lights having wavelengths corresponding to the first to n-th channels are multiplexed, and an pumping light having a wavelength spaced apart from the n-th channel by 2 THz or more toward the longer wavelength side is combined with the multiplexed light, thereby forming the pumping light source. The pumping lights having wavelengths corresponding to the channels other than (n-1)-th and (n-2)-th channels may be multiplexed, thereby forming the pumping light source. The pumping lights having wavelengths corresponding to the channels other than (n-2)-th and (n-3)-th channels may be multiplexed, thereby forming the pumping light source.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of application Ser. No. 10/824,402,filed Mar. 17, 2000, which is a divisional of Ser. No. 10/120,173, filedApr. 11, 2002; which is a continuation of application Ser. No.09/886,212 filed Jun. 22, 2001, now U.S. Pat. No. 6,654,162; which is acontinuation of application Ser. No. 09/527,748 filed Mar. 17, 2000, nowU.S. Pat. No. 6,292,288; which is a Continuation-in-Part ofPCT/JP99/03944 filed Jul. 23, 1999.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a Raman amplifier capable of being usedfor amplification of optical signal in various optical communicationsystems, and optical repeater and Raman amplification method using sucha Raman amplifier, and more particularly, it relates to Raman amplifier,optical repeater and Raman amplification method suitable foramplification of wavelength division multiplexing signals.

2. Related Background Art

Almost all of optical amplifiers used with present optical fibercommunication systems are rare earth doped fiber amplifiers.Particularly, erbium doped optical fiber amplifier (referred to as“EDFA” hereinafter) using Er (erbium) doped fibers have been usedfrequently. However, a practical gain wavelength band of the EDFA has arange from about 1530 nm to 1610 nm (refer to “Electron. Lett,” vol. 33,no. 23, pp. 1967-1968). Further, the EDFA includes gain havingwavelength dependency, and, thus, when it is used with wavelengthdivision multiplexing signals, difference in gain is generated independence upon a wavelength of optical signal. FIG. 23 shows an exampleof gain wavelength dependency of the EDFA. Particularly, in wavelengthbands smaller than 1540 nm and greater than 1560 nm, change in gainregarding the wavelength is great. Accordingly, in order to obtain givengain (in almost cases, gain deviation is within 1 dB) in the entire bandincluding such wavelength, a gain flattening filter is used.

The gain flattening filter is a filter designed so that loss isincreased in a wavelength having great gain, and the loss profile has ashape substantially the same as that of the gain profile. However, Asshown in FIG. 24, in the EDFA, when magnitude of average gain ischanged, since the gain profile is also changed as shown by curves a, band c, in this case, the optimum loss profile of the gain flatteningfilter is also changed. Accordingly, when the flattening is realized bya gain flattening filter having fixed loss profile, if the gain of theEDFA is changed, the flatness will be worsened.

On the other hand, among optical amplifiers, there is an amplifierreferred to as a Raman amplifier utilizing Raman scattering of anoptical fiber (refer to “Nonlinear Fiber Optics, Academic Press). TheRaman amplifier has peak gain in frequency smaller than frequency ofpumping light by about 13 THz. In the following description, it isassumed that pumping light having 1400 nm band is used, and thefrequency smaller by about 13 THz will be expressed as wavelength longerby about 100 nm. FIG. 25 shows wavelength dependency of gain whenpumping light having central wavelength of 1450 nm is used. In thiscase, peak of gain is 1550 nm and a band width within gain deviation of1 dB is about 20 nm. Since the Raman amplifier can amplify anywavelength so long as an pumping light source can be prepared,application of the Raman amplifier to a wavelength band which could notbe amplified by the EDFA has mainly be investigated. On the other hand,the Raman amplifier has not been used in the gain band of the EDFA,since the Raman amplifier requires greater pump power in order to obtainthe same gain as that of the EDFA. When the pumping light having greatpower is input to a fiber to increase the gain, stimulated Brillouinscattering may be generated. Increase amplification noise caused by thestimulated Brillouin scattering is one of the problems which makes itdifficult to use the Raman amplifier. the Japanese Patent Laid-open No.2-12986 (1990) discloses an example of a technique for suppressing thestimulated Brillouin scattering in the Raman amplifier.

Further, the Raman amplifier has polarization dependency of gain andamplifies only a component (among polarized wave components) coincidedwith the polarized wave of the pumping light. Accordingly, it isrequired for reducing unstability of gain due to polarizationdependency, and, to this end, it is considered that a polarizationmaintaining fiber is used as a fiber for amplifying or an pumping lightsource having random polarization condition.

Furthermore, enlargement of the gain band is required in the Ramanamplifier. To this end, Japanese Patent Publication No. 7-99787 (1995)teaches in FIG. 4 that the pumping light is multiplexed with appropriatewavelength interval. However, this patent does not disclose concretevalues of the wavelength interval. According to a document (K. Rottwitt,OFC98, PD-6), a Raman amplifier using a plurality of pumping lightshaving different wavelengths was reported; however, attempt in theviewpoint of the fact that the gain deviation is reduced below 1 dB wasnot considered.

On the other hand, there is an optical repeater for simultaneouslycompensating for transmission loss and chromatic dispersion in anoptical fiber transmission line, which optical repeater is constitutedby combination of an Er doped fiber amplifier (EDFA) and a dispersioncompensating fiber (DCF). FIG. 46 shows a conventional example in whicha dispersion compensating fiber A is located between two Er doped fiberamplifiers B and C. The first Er doped fiber amplifier B serves toamplify optical signal having low level to a relatively high level andhas excellent noise property. The second Er doped fiber amplifier Cserves to amplify the optical signal attenuated in the dispersioncompensating fiber A to the high level again and has a high outputlevel.

By the way, on designing the optical repeater, it is required that arepeater input level, a repeater output level and a dispersioncompensating amount (loss in the dispersion compensating fiber A) be setproperly, and, there is limitation that the input level of thedispersion compensating fiber A has an upper limit, because, when theinput power to the dispersion compensating fiber A is increased,influence of non-linear effect in the dispersion compensating fiber A isalso increased, thereby deteriorating the transmission wave formconsiderably. The upper limit value of the input power to the dispersioncompensating fiber A is determined by self phase modulation (SPM) in onewave transmission and by cross phase modulation (XPM) in WDMtransmission. Thus, regarding the optical repeater, an optical repeaterhaving excellent gain flatness and noise property must be designed inconsideration of the several variable factors.

FIG. 47 shows a signal level diagram in the repeater. Gain G1 [dB] ofthe first Er doped fiber amplifier B is set to a difference between aninput level Pin [dB] of the repeater and an input upper limit value Pd[dB] to the dispersion compensating fiber A. Gain G2 [dB] of the secondEr doped fiber amplifier C is set to (Gr+Ld−G1) [dB] from loss Ld [dB]in the dispersion compensating fiber A, gain Gr [dB] of the repeater andthe gain G1 [dB] of the first Er doped fiber amplifier B. Since thesedesign parameters are varied for each system, the values G1 [dB] and G2[dB] are varied for each system, and, accordingly, the Er doped fiberamplifiers B, C must be re-designed for each system. The noise propertyin such a system is deeply associated with the loss Ld [dB] in thedispersion compensating fiber A, and it is known that the greater theloss the more the noise property is worsened. Further, at present, a gapfrom the designed value in loss in the transmission line and the loss inthe dispersion compensating fiber A are offset by changing the gains ofEr doped fiber amplifiers B, C. In this method, the gain of Er dopedfiber amplifier B and C are off the designed value, thus the gainflatness is worsened. A variable attenuator may be used to offset thegap from the designed value of loss. In this method, although the gainflatness is not changed, an additional insertion loss worsens the noiseproperty.

In the optical fiber communication system, although the Er doped opticalfiber amplifiers have widely been used, the Er doped optical fiberamplifier also arises several problems. Further, the Raman amplifieralso has problems that, since output of ordinary semiconductor laser isabout 100 to 200 mW, gain obtained is relatively small, and that thegain is sensitive to change in power or wavelength of the pumping light.So that, when a semiconductor laser of Fabry-Perot type havingrelatively high output is used, noise due to gain fluctuation caused byits mode hopping becomes noticeable, and that, when the magnitude of thegain is adjusted, although drive current of the pumping laser must bechanged, if the drive current is changed, since the fluctuation in thecentral wavelength is about 15 nm at the maximum, the wavelengthdependency of gain will be greatly changed. Further, such shifting ofthe central wavelength is not preferable because such shifting causeschange in joining loss of a WDM coupler for multiplexing the pumpinglight. In addition, the optical repeater also has a problem that the Erdoped optical fiber amplifiers B, C must be re-designed for each system.Further, the deterioration of the noise property due to insertion of thedispersion compensating fiber is hard to be eliminated in the presentsystems.

In a Raman amplification method for amplifying optical signal by using astimulated Raman scattering phenomenon, a communication optical fiber isused as an optical fiber acting as an amplifying medium, and, in adistributed amplifying system, a wavelength of pumping light and awavelength of the optical signal are arranged in 1400 nm-1600 nm bandhaving low loss and low wavelength dependency within a wide band of thecommunication optical fiber. In this case, regarding the loss ofwavelength dependency of the optical fiber as the amplifying medium, adifference between maximum and minimum values is below about 0.2 dB/kmin the above band, even in consideration of loss caused by hydroxyl ion(OH) having peak at 1380 nm. Further, even if each pumping powers in amulti-wavelength pumping system are not differentiated according to thewavelength dependence of the loss, the gain of the signals amplified bythe pumping lights are substantially the same, there is no problem inpractical use.

On the other hand, in a Raman amplifier operating as a discreteamplifier such as EDFA (rare earth doped fiber amplifier) it isnecessary to pay attention to the package of the amplifier fiber, for alength of the fiber is about 10 km to about several tens of kilometersin order to obtain the required gain. Thus, it is convenient that thelength of the fiber is minimized as less as possible. Although thelength of the fiber can be shortened by using an optical fiber havinggreat non-linearity, in the optical fiber having great non-linearity, itis difficult to reduce the transmission loss caused by (OH) generallyhaving a band of 1380 nm, and Rayleigh scattering coefficient becomesgreat considerably in comparison with the communication fiber, with theresult that the difference between the maximum and minimum values offiber loss within the above-mentioned wavelength range becomes verygreat such as 1.5 to 10 dB/km. This means that, when the optical fiberhaving length of 3 km is used, the loss difference due to the wavelengthof the pumping light becomes 4.5 dB to 30 dB. Thus, the wavelengthdivision multiplexing signals cannot be uniformly amplified by using thepumping lights having the same intensities.

As one of means for multiplexing the number of pumping lights, there isa wavelength combiner of Mach-Zehnder interferometer type. Since theMach-Zehnder interferometer has periodical response property regardingfrequency, the wavelength of the pumping light must be selected amongwavelengths having equal intervals in frequency. Accordingly, thewavelength combiner of Mach-Zehnder interferometer type has limitationin degrees of freedom of wavelength setting, but has an advantage that,when a device of waveguide type or fiber fusion type, if the number ofwavelength division multiplexing is increased, insertion loss is notchanged substantially.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a Raman amplificationmethod capable of uniformly amplifying wavelength division multiplexingsignals and suitable to be incorporated as a unit.

Another object of the present invention is to provide a Raman amplifierwhich can obtain required gain and can reduce wavelength dependency ofgain to the extent that usage of a gain flattening filter is notrequired and which can be used in a band of EDFA.

A further object of the present invention is to apply the Ramanamplifier to an optical repeater constituted by an Er doped fiberamplifier (EDFA) and a dispersion compensating fiber (DCF) thereby toprovide an optical repeater in which the EDFA is not required to bere-designed for each system and which can compensate dispersion intransmission line loss and/or DCF loss without deteriorating property ofthe optical repeater.

Further, by Raman-amplifying the DCF, the deterioration of noiseproperty due to insertion of the DCF which could not avoided in theconventional techniques is reduced.

An example of a Raman amplifier according to the present invention isshown in FIGS. 1, 2 and 3. When a small-sized semiconductor laser 3 ofFabry-Perot type having relatively high output is used in a pumpingmeans 1, relatively high gain can be obtained, and, since thesemiconductor laser 3 of Fabry-Perot type has a wide line width of anoscillating wavelength, occurrence of stimulated Brillouin scatteringdue to the pumping light can be eliminated substantially. On the otherhand, when a semiconductor laser of DFB type or DBR type, or a MaterOscillator Power Amplifier (MOPA) is used in the pumping means 1, sincea fluctuation range of the oscillating wavelength is relatively small, again configuration is not changed by a driving condition. Further,occurrence of stimulated Brillouin scattering can be suppressed byeffecting modulation.

Further, by selecting the interval between the central wavelengths ofthe pumping light to a value greater than 6 nm and smaller than 35 nm,the wavelength dependency of gain can be reduced to the extent that thegain flattening filter is not required. The central wavelength λ c inthis case is a value defined regarding the single pumping light and isrepresented by the following equation when it is assumed that awavelength of an i-th longitudinal mode of laser oscillating is λi andlight power included in that mode is Pi:

The reasons why the interval between the central wavelengths of thepumping light is selected to a value greater than 6 nm is that theoscillating band width of the semiconductor laser 3 of Fabry-Perot typeconnected to an external resonator 5 having narrow reflection band widthis about 3 nm as shown in FIG. 12, and that a WDM coupler 11 (FIGS. 1, 2and 3) for combining the pumping lights is permitted to have certainplay or margin in wavelength interval between the pumping lights inorder to improve wave combining efficiency. The WDM coupler 11 isdesigned so that lights having different wavelengths are received bydifferent ports and the incident lights are joined at a single outputport substantially without occurring of loss of lights. However,regarding light having intermediate wavelength between the designedwavelengths, the loss is increased even whichever port is used. Forexample, in a certain WDM coupler 11, a width of a wavelength band whichincreases the loss is 3 nm. Accordingly, in order that the band of thesemiconductor laser 3 is not included within said band, as shown in FIG.12, a value 6 nm obtained by adding 3 nm to the band width of thesemiconductor laser 3 is proper for low limit of the interval betweenthe central wavelengths of the pumping light. On the other hand, asshown in FIG. 13A, if the interval between the central wavelengths ofthe semiconductor laser 3 is greater than 35 nm, as shown in FIG. 13B, again valley is created at an intermediate portion of the Raman gain bandobtained by the pumping lights having adjacent wavelengths, therebyworsening the gain flatness. The reason is that, regarding the Ramangain obtained by the single pumping light, at a position spaced apartfrom the gain peak wavelength by 15 nm to 20 nm, the gain is reduced tohalf. Accordingly, by selecting the interval of the central wavelengthsof the pumping light to the value greater than 6 nm and smaller than 35nm, the wavelength dependency of gain can be reduced to the extent thatthe gain flattening filter is not required.

According to a second aspect of the present invention, since a Ramanamplifier has a control means 4 for monitoring input light or outputlight with respect to the Raman amplifier and for controlling pumppowers of the pumping means 1 on the basis of a monitored result tomaintain output light power of the Raman amplifier to a predeterminedvalue, given output can be obtained regardless of fluctuation of inputsignal power to the Raman amplifier and/or dispersion in loss of a Ramanamplifier fiber.

According to a third aspect of the present invention, since a Ramanamplifier has a controlling means 4 for flattening Raman gain, the gaincan be flattened. Particularly, as shown in FIG. 16, by monitoringlights having wavelengths obtained by adding about 100 nm to thewavelengths of the pumping lights, respectively, and by controlling thepowers of the pumping lights to order or align the monitored lightpowers, the gain can be flattened. Further, since a wavelengthstabilizing fiber grating (external resonator 5) which will be describedlater can suppress the shift in the central wavelength due to change indrive current of a Fabry-Perot type semiconductor laser, it can also beused as a means for permitting control of the gain.

According to a fourth aspect of the present invention, since a Ramanamplifier has a control means 49 for monitoring input signal power andoutput signal power and for controlling pumping light power to make aratio between the input signal power and the output signal powerconstant thereby to maintain gain of the Raman amplifier to apredetermined value, given output can be obtained regardless offluctuation of the input signal power to the Raman amplifier and/ordispersion in loss of a Raman amplifier fiber.

According to a fifth aspect of the present invention, in a Ramanamplifier, since an optical fiber having non-linear index n2 ofrefraction of 3.5E-20 [m2/W] or more is used as an optical fiber 2,adequate amplifying effect can be obtained, from the result ofinvestigation.

According to a sixth aspect of the present invention, in a Ramanamplifier, since the optical fiber 2 exists as a part of a transmissionfiber for propagating the optical signal, the amplifier can beincorporated into the transmission optical fiber as it is.

According to a seventh aspect of the present invention, a Ramanamplifier utilizes a part of a dispersion managed transmission line andcan constitute an amplifier as it is as a amplifying medium.

According to an eighth aspect of the present invention, in a Ramanamplifier, since the optical fiber 2 exists as an amplifier fiberprovided independently from a transmission fiber for propagating theoptical signal and inserted into the transmission fiber, an opticalfiber suitable for Raman amplification can easily be used for theoptical fiber 2 and the chromatic dispersion compensating fiber caneasily be utilized, and a discrete amplifier can be constituted.

In an optical repeater according to the present invention, since is lossof an optical fiber transmission line 8 is compensated by using theRaman amplifier, an optical repeater having the function of the Ramanamplifier can be provided.

Among optical repeaters of the present invention, in a repeater in whichrare earth doped fiber amplifier(s) 10 is (are) provided at preceding orfollowing stage or at both stages of the Raman amplifier, since the lossof the optical fiber transmission line 8 is compensated by the Ramanamplifier 9 and the rare earth doped fiber amplifier(s) 10, desiredamplifying property suitable for various transmission systems can beobtained.

Among optical repeaters of the present invention, in accordance with anarrangement in which the Raman amplifier 9 and the rare earth dopedfiber amplifier 10 are combined, a repeater adapted to various systemscan be obtained. This fact will be explained hereinbelow as an examplethat DCF is used as the amplifier fiber of the Raman amplifier 9. FIG.17 shows design parameters of a conventional optical repeater, and thegains G1, G2 are varied for each system. Further, it is not inevitablethat input of the repeater and loss of the DCF are fluctuated byscattering in repeater spacing and scattering in the DCF. Suchfluctuation is directly associated with the fluctuation of the gain ofthe EDFA, which fluctuation of the gain leads to deterioration offlatness. FIG. 18 schematically shows a relationship between theflatness and the gain of the EDFA. Since the flatness is optimized bylimiting the used band and the average gain, if the average gain isdeviated from the optimum point, the wavelength dependency of gain ischanged to worsen the flatness. In order to avoid the deterioration ofthe flatness, the gain of the EDFA must be kept constant.Conventionally, a variable attenuator has been used as a means forcompensating the fluctuation in input level and loss of the DCF. FIG.19A shows an example that an attenuating amount of the variableattenuator is adjusted in accordance with the fluctuation of the inputlevel to control the input level to the DCF to be kept constant and FIG.19B shows an example that an attenuating amount is adjusted inaccordance with the fluctuation in loss of the DCF to controlintermediate loss to be kept constant. In both examples, the gains oftwo amplifiers are constant. However, in these methods, since uselessloss is added by the variable attenuator, there is an disadvantage inthe viewpoint of noise property. In the present invention, bycompensating the change in design parameters of the repeater by theRaman amplification of the DCF, the gain of the EDFA is kept constant,requirement that the EDFA be re-designed for each system is eliminated,and the scattering in repeater spacing and scattering in the DCF can becompensated without sacrificing the flatness and the noise property.FIG. 20 shows design values of the EDFA when the Raman amplification ofthe DCF is applied to the specification of the repeater of FIG. 17. Byselecting the Raman gain of the DCF appropriately, the properties of theEDFA required for three specifications can be made common. Further, asshown in FIGS. 21A and 21B, the fluctuation in input level and in lossof the DCF can be compensated by changing the Raman gain withoutchanging the gain of the EDFA. In any cases, the Raman gain is adjustedso that the output level of the DCF becomes constant, while keeping thegain of the EDFA constant. Further, by compensating the loss of the DCFitself by the Raman amplification, the deterioration of the noiseproperty due to insertion of the DCF which could not avoided in theconventional techniques can be reduced. FIG. 37 shows measured values ofa deteriorating amount of the noise figure when the DCF is inserted andof a deteriorating amount of the noise figure when the Raman amplifierusing the same DCF is inserted.

In the optical repeater according to the present invention, when a Ramanamplifier using an pumping light source in which wavelengths are notcombined is provided, although an operating wavelength range isnarrower, a construction can be simplified and the same property as theaforementioned optical repeaters can be obtained, except for the bandwidth, in comparison with an optical repeater having a Raman amplifierpumped by a plurality of wavelengths. FIGS. 38 and 39 show measuredexamples of the optical repeater using the Raman amplifier pumped by thepumping light source in which wavelengths are not combined and of theoptical repeater using the Raman amplifier pumped by the plurality ofwavelengths.

In the Raman amplifier according to the present invention, when adifference between maximum and minimum values of the central wavelengthof the pumping light is within 100 nm, overlapping between the pumpinglight and the optical signal can be prevented to prevent wave formdistortion of the optical signal. If the wavelength of the pumping lightis similar to the wavelength of the optical signal, since the wave formof the optical signal may be deteriorated, the wavelength of the pumpinglight and the wavelength of the optical signal must be selected so thatthey are not overlapped. However, in a case where the pumping light hasband of 1.4 μm, when the difference between the maximum and minimumvalues of the central wavelength of the pumping light is smaller than100 nm, as shown in FIG. 14, since the difference between the centralwavelength of the gain caused by one pumping light and the wavelength ofsuch pumping light is about 100 nm, the overlapping between thewavelength of the pumping light and the wavelength of the optical signalcan be prevented.

In the Raman amplifier according to the present invention, when thepumping lights having adjacent wavelengths are propagated through theoptical fiber 2 toward two different directions so that the opticalsignal pumped bi-directionally, the wavelength property required in theWDM coupler 11 shown in FIG. 1 and FIGS. 2 and 3 can be softened. Thereason is that, as shown in FIG. 15, in all of pumping lights from bothdirections, although the central wavelengths become λ₁, λ₂, λ₃, λ₄ andthe interval is greater than 6 nm and smaller than 35 nm, whenconsidering the pumping lights from only one direction, the centralwavelengths become λ₁ and λ₃ or λ₂ and λ₄ and the wavelength intervalincreased to twice, with the result that the property required in theWDM coupler 11 can have margin or play.

In the Raman amplifier according to the present invention, when thewavelength stabilizing external resonator 5 such as fiber grating isprovided at the output side of the semiconductor laser 3 of Fabry-Perottype, noise due to fluctuation of caused by mode hopping of thesemiconductor laser 3 of Fabry-Perot type can be suppressed. Inconsideration of one pumping light source, it makes the bandwidthnarrower to connect the wavelength stabilizing external resonator 5 tothe semiconductor laser 3. Since it also results the smaller wavelengthinterval in case of combining pumping light sources by the WDM coupler11 (FIGS. 1, 2 and 3), pumping light having higher output and wider bandcan be generated.

In the Raman amplifier according to the present invention, when thepumping light of the semiconductor laser 3 is used to bepolarization-combined for each wavelength, not only polarizationdependency of gain can be eliminated but also the pump power launchedinto the optical fiber 2 can be increased. In the Raman amplification,only the components matched with the polarized of the pumping light canbe given the gain. When the pumping light is linear-polarized and theamplifier fiber is not a polarization maintaining fiber, the gain ischanged due to fluctuation of relative state of polarization of thesignal and the pumping light. Therefore, the polarization dependence ofgain can be eliminated by combining two linear-polarized pumping lightsso that the polarization planes are perpendicular to each other.Further, it increases the pumping light power launched into the fiber.

In the Raman amplifier according to the present invention, in the casewhere a wavelength combiner of planar lightwave circuit based on aMach-Zehnder interferometer is used as a means for wave-combining MOPAor a semiconductor laser of Fabry-Perot type, DFB type or DBR typehaving a plurality of wavelengths, even when the number of thewavelengths to be combined is large, the wave-combination can beachieved with very low loss, and pumping light having high output can beobtained.

In the Raman amplifier according to the present invention, as shown inFIG. 6, when a polarization plane rotating means 7 for rotating thepolarization plane by 90 degrees is provided so that the optical fiber 2simultaneously includes the plurality of pumping lights generated by thepumping means 1 and the pumping lights having a orthogonal state ofpolarization to the pumping lights generated by the pumping means 1, inprinciple, given gain can always be obtained regardless of the state ofpolarization of the optical signal, with the result that the band of theRaman gain can be widened.

In the optical repeater according to the present invention, when theRaman amplification is utilized by coupling residual pumping light ofthe Raman amplifier to the optical fiber transmission line 8, a part ofloss of the optical fiber transmission line 8 can be compensated.

In the optical repeater according to the present invention, when theresidual pumping light of the Raman amplifier is utilized as pumpinglight for the rare earth doped fiber amplifier 10, the number of thesemiconductor lasers to be used can be reduced.

In the optical repeater according to the present invention, when adispersion compensating fiber is used as the optical fiber 2 of theRaman amplifier 9, the chromatic dispersion of the optical fibertransmission line 8 can be compensated by the Raman amplifier 9, and apart or all of the losses in the optical fiber transmission line 8 andthe amplifier fiber 2 can be compensated.

According to a twenty-ninth aspect of the present invention, in a Ramanamplification method by stimulated Raman scattering in an optical fiberthrough which two or more pumping lights having different centralwavelengths and said optical signals are propagated, the pumping powerlaunched into said optical fiber increases as the central wavelengths ofsaid pumping lights is shorter.

According to a thirtieth aspect of the present invention, in a Ramanamplification method by stimulated Raman scattering in an optical fiberthrough which two or more pumping lights having different centralwavelengths and said optical signals are propagated, total pumping poweron the shorter wavelength side with respect to the center between theshortest and longest central wavelengths among said two or more pumpinglights is greater than on the longer side.

According to a thirty-first aspect of the present invention, in a Ramanamplification method by stimulated Raman scattering in an optical fiberthrough which three or more pumping lights having different centralwavelengths and said optical signals are propagated, the number of thepumping light sources on the shorter wavelength side with respect to thecenter between the shortest and longest central wavelengths among saidthree or more pumping lights is greater than on the longer side, and thetotal pumping power on the shorter wavelength side is greater than onthe longer side.

To achieve the above object, according to thirty-second to thirty-fourthaspects of the present invention, when the shortest pumping wavelengthis defined as a first channel and, from the first channel, at respectiveintervals of about 1 THz toward the longer wavelength, second to n-thchannels are defined, pumping lights having wavelengths corresponding tothe first to n-th channels are multiplexed and pumping light having awavelength spaced apart from the n-th channel by 2 THz or more towardthe longer wavelength is combined with the multiplexed light, and lightobtained in this way is used as the pumping light of the Ramanamplifier. When the shortest pumping wavelength is defined as the firstchannel and, from the first channel, at respective intervals of about 1THz toward the longer wavelength, the second to n-th channels aredefined, light obtained by combining all of the wavelengthscorresponding to the channels other than (n-1)-th and (n-2)-th channelsis used as the pumping light of the Raman amplifier. Alternatively,light obtained by combining all of the wavelengths corresponding to thechannels other than (n-2)-th and (n-3)-th channels is used as thepumping light of the Raman amplifier.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a first embodiment of a Raman amplifieraccording to the present invention;

FIG. 2 is a diagram showing a second embodiment of a Raman amplifieraccording to the present invention;

FIG. 3 is a diagram showing a third embodiment of a Raman amplifieraccording to the present invention;

FIG. 4 is a diagram showing a first example of a controlling means inthe Raman amplifier according to the present invention;

FIG. 5 is a diagram showing a second example of a controlling means inthe Raman amplifier according to the present invention;

FIGS. 6A and 6B are diagrams showing different examples of apolarization plane rotating means in the Raman amplifier according tothe present invention;

FIG. 7 is a diagram showing a first embodiment of an optical repeateraccording to the present invention;

FIG. 8 is a diagram showing a second embodiment of an optical repeateraccording to the present invention;

FIG. 9 is a diagram showing a third embodiment of an optical repeateraccording to the present invention;

FIG. 10 is a diagram showing a fourth embodiment of an optical repeateraccording to the present invention;

FIG. 11 is a diagram showing a fifth embodiment of an optical repeateraccording to the present invention;

FIG. 12 is an illustration of why a wavelength interval of pumping lightis selected to be greater than 6 nm;

FIGS. 13A and 13B are illustrations of why a wavelength interval ofpumping light is selected to be smaller than 35 nm;

FIG. 14 is an illustration of why a difference between a longestwavelength and a shortest wavelength is selected to be smaller than 100nm;

FIG. 15 is an illustration of an example of wavelength arrangement ofpumping lights in bidirectional pumping;

FIG. 16 is an illustration of how a condition that a gain over aspecified band is flattened by controlling pumping light power;

FIGS. 17A and 17B are illustrations of properties associated withdesigning of the optical repeater;

FIG. 18 is an illustration of a relationship between fluctuation in gainof EDFA and deterioration of flatness;

FIG. 19A is an illustration of how a condition that fluctuation in inputlevel due to a variable attenuator is compensated, and FIG. 19B is anexplanatory view showing a condition that fluctuation in loss of DCF dueto a variable attenuator is compensated;

FIGS. 20A and 20B are illustrations of properties associated withdesigning of the optical repeater utilizing a Raman amplification inDCF;

FIG. 21A is an illustration of how a condition that fluctuation in inputlevel due to the Raman amplification is compensated, and FIG. 21B is anexplanatory view showing a condition that fluctuation in loss of DCF dueto the Raman amplification is compensated;

FIG. 22 is an illustration of different examples of output spectrum inthe Raman amplifier;

FIG. 23 is an illustration of wavelength dependency of gain due to EDFA;

FIG. 24 is an illustration of fluctuation in gain due to EDFA;

FIG. 25 is an illustration of wavelength dependency of gain due to Ramanamplification;

FIG. 26 is a diagram showing a control method for controlling outputlight power by monitoring input light;

FIG. 27 is a diagram showing a control method for controlling outputlight power by monitoring output light;

FIG. 28 is a diagram showing a control method for controlling outputlight power by monitoring input light and output light;

FIG. 29 is a diagram showing a first example of a method for obtainingRaman gain by coupling residual pumping light of the Raman amplifier toan optical fiber transmission line;

FIG. 30 is a diagram showing a second example of a method for obtainingRaman gain by coupling residual pumping light of the Raman amplifier toan optical fiber transmission line;

FIG. 31 is a diagram showing a third example of a method for obtainingRaman gain by coupling residual pumping light of the Raman amplifier toan optical fiber transmission line;

FIG. 32 is a diagram showing a fourth example of a method for obtainingRaman gain by coupling residual pumping light of the Raman amplifier toan optical fiber transmission line;

FIG. 33 is a diagram showing a first example of a method for utilizingthe residual pumping light of the Raman amplifier as pumping light ofEDFA;

FIG. 34 is a diagram showing a second example of a method for utilizingthe residual pumping light of the Raman amplifier as pumping light ofEDFA;

FIG. 35 is a diagram showing a third example of a method for utilizingthe residual pumping light of the Raman amplifier as pumping light ofEDFA;

FIG. 36 is a diagram showing a fourth example of a method for utilizingthe residual pumping light of the Raman amplifier as pumping light ofEDFA;

FIG. 37 is an illustration of deterioration of noise figure due toinsertion of a dispersion compensating fiber;

FIG. 38 is an illustration of the number of pumping wavelengths of theRaman amplifier and property of the repeater;

FIG. 39 is an illustration of the number of pumping wavelengths of theRaman amplifier and property of the repeater;

FIG. 40 is a diagram of an optical repeater in which a plurality ofRaman amplifiers are connected in a multi-stage fashion;

FIG. 41 is a diagram showing an example of a pumping means having asingle pumping light source;

FIG. 42 is a diagram showing another example of a pumping means having asingle pumping light source;

FIG. 43 is a diagram showing an example of a pumping means having twopumping light sources;

FIG. 44 is a diagram showing another example of a pumping means havingtwo pumping light sources;

FIG. 45 is a diagram of a Raman amplifier in which a dispersioncompensating fiber is used as an amplifier fiber;

FIG. 46 is a diagram showing an example of a conventional opticalrepeater;

FIG. 47 is an illustration of signal level diagram in the opticalrepeater of FIG. 46;

FIG. 48 is a diagram showing a case where SMF and a fiber havingdispersion smaller than −20/ps/nm/km are used as the amplifier fiber;

FIG. 49 is an illustration of an example of an arrangement for carryingout a Raman amplification method according to the present invention;

FIG. 50 is an explanatory view showing an pumping light source of FIG.49;

FIG. 51 is a view showing gain profile of a optical signalRaman-amplified by the Raman amplifying method according to the presentinvention;

FIG. 52 is a view showing gain profile of a optical signalRaman-amplified by a conventional method;

FIG. 53 is an explanatory view showing another example of an arrangementfor carrying out a Raman amplification method according to the presentinvention;

FIGS. 54A and 54B are views showing Raman gain profiles when an intervalbetween pumping light is selected to 4.5 THz and 5 THz, respectively andDSF is used as the amplifier fiber;

FIG. 55 is a view showing Raman gain profile when the interval betweenpumping light is selected to 4.5 THz and three wavelengths are used;

FIG. 56 is a view showing Raman gain profile when the interval betweenpumping light is selected to 2.5 THz and 4.5 THz and three wavelengthsare used;

FIG. 57 is a view showing performance of Raman gain curves when theintervals of the pumping lights are equidistant and the peak gains areadjusted to 10 dB on the same;

FIG. 58 is a view showing performance of Raman gain curves when theintervals of the pumping lights are equidistant and the gains generatedby the respective pumping lights are adjusted so that the gain curvesare flattened;

FIG. 59 is a view showing performance of Raman gain curves when theintervals of the pumping lights are equidistant of 1 THz and the numberof multiplexing is changed;

FIG. 60 is a schematic diagram of a sixth embodiment of a Ramanamplifier according to the present invention;

FIG. 61 is a view showing gain profile when a pumping light source ofFIG. 60 is used;

FIG. 62 is an enlarged view of total gain shown in FIG. 61;

FIG. 63 is a view showing Raman gain profile when a wavelength of asixth channel is a wavelength spaced apart from a fifth channel by 2.5THz toward the longer wavelength in the Raman amplifier shown in FIG.60;

FIG. 64 is an enlarged view of total gain shown in FIG. 63;

FIG. 65 is a schematic diagram of a seventh embodiment of a Ramanamplifier according to the present invention;

FIG. 66 is a view showing Raman gain profile when an pumping lightsource of FIG. 65 is used;

FIG. 67 is an enlarged view of total gain shown in FIG. 66;

FIG. 68 is an explanatory view showing an eighth embodiment of a Ramanamplifier according to the present invention;

FIG. 69 is a view showing Raman gain profile when an pumping lightsource of FIG. 68 is used;

FIG. 70 is an enlarged view of total gain shown in FIG. 69;

FIG. 71 is a view showing Raman gain profile when eleven channels areused among thirteen channels at interval of 1 THz from 211 THz to 199THz and pumping lights other than 201 THz and 200 THz are used;

FIG. 72 is an enlarged view of total gain shown in FIG. 71;

FIG. 73 is a view showing Raman gain profile when eleven channels areused among thirteen channels at interval of 1 THz from 211 THz to 199THz and pumping lights other than 202 THz and 201 THz are used; and

FIG. 74 is an enlarged view of total gain shown in FIG. 73.

BEST MODE FOR CARRYING OUT THE INVENTION First Embodiment of RamanAmplifier

FIG. 1 shows a first embodiment of a Raman amplifier according to thepresent invention. The Raman amplifier comprises a signal input fiber12, an amplifier fiber (optical fiber) 2, a WDM coupler 13, a pumpingmeans 1, a tap coupler for monitoring 14, a monitor signal detecting andLD control signal generating circuit 15, a signal output fiber 16, and apolarization independent isolator 25. The tap coupler for monitoring 14and the monitor signal detecting and LD control signal generatingcircuit 15 constitute a controlling means 4.

The pumping means 1 includes semiconductor lasers 3 (3 ₁, 3 ₂, 3 ₃, 3 ₄)of Fabry-Perot type, wavelength stabilizing fiber ratings (externalresonators) 5 (5 ₁, 5 ₂, 5 ₃, 5 ₄), polarization coupler (polarizationbeam combiner) 6 (6 ₁, 6 ₁), and a WDM coupler 11. Oscillatingwavelengths of the semiconductor lasers 3 ₁, 3 ₂ and permeationwavelengths of the fiber gratings 5 ₁, 5 ₂ are the same wavelength λ₁,and central wavelengths of the semiconductor lasers 3 ₃, 3 ₄ andpermeation wavelengths of the fiber gratings 5 ₃, 5 ₄ are the samewavelength λ₂, and the oscillating wavelengths of the semiconductorlasers 3 ₁, 3 ₂, 3 ₃, 3 ₄ can be subjected to the action of thewavelength stabilizing fiber gratings 5 ₁, 5 ₂, 5 ₃, 5 ₄ so that thecentral wavelengths are stabilized to λ₁, λ₂. Further, a wavelengthinterval between the wavelength λ₁ and the wavelength λ₂ is selected tobe greater than 6 nm and smaller than 35 nm.

Pumping lights generated by the semiconductor lasers 3 ₁, 3 ₂, 3 ₃, 3 ₄are polarization-combined by the polarization coupler 6 for eachwavelength λ₁, λ₂, and output lights from the polarization coupler 6 arecombined by the WDM coupler 11 to obtain output light of the pumpingmeans 1. Polarization maintaining fibers 17 are connected between thesemiconductor lasers 3 and the polarization coupler 6 to obtain twopumping lights having different polarization planes. The output light ofthe pumping means 1 is coupled to the amplifier fiber 2 via the WDMcoupler 13; on the other hand, an optical signal (wavelength divisionmultiplexing signal) is incident on the amplifier fiber 2 from theoptical signal input fiber 12 and then is combined with the pumpinglight of the pumping means 1 in the amplifier fiber 2 to beRaman-amplified, and the Raman-amplified light is passed through the WDMcoupler 13 and is sent to the monitor light branching coupler 14, wherea part of the light is branched as a monitor signal, and the other isoutputted to the optical signal output fiber 16. The monitor signal ismonitored in the monitor signal detecting and LD control signalgenerating circuit 15, and the circuit 15 generate a signal forcontrolling drive currents for the semiconductor lasers 3 so that gaindeviation in the signal wavelength band becomes small.

The amplifier fiber 2 may be a special fiber suitable for Ramanamplification (for example, a fiber having non-linear index ofrefraction n2 of 3.5 E-20 [m2/W] or more) or may be an extension of thesignal input fiber 12 by which the optical signal is received. Further,as shown in FIG. 48, RDF (reverse dispersion fiber) having dispersion ofless than −20 ps/nm per 1 km may be connected to SMF so that theamplifier fiber can also act as a transmission line. (Generally, sinceRDF has dispersion of less than −20 ps/nm, RDF having a lengthsubstantially the same as or greater, by twice, than a length of SMF maybe used.) In such a case, preferably, it is designed so that the Ramanamplifying pumping light is propagated from the RDF toward the SMF.

In the Raman amplifier, the amplifier fiber 2 may be connected to andinserted into a transmission fiber (not shown) to which the opticalsignal is transmitted, and the amplifier fiber 2, pumping means 1, WDMcoupler 13, monitor light branching coupler 14, and monitor signaldetecting and LD control signal generating circuit 15 may beincorporated to constitute a concentrated Raman amplifier.

FIG. 22 shows the output spectrum of the Raman amplifier of FIG. 1. Inthis experiment, the wavelength of 1435 nm and 1465 nm were used as thepumping wavelengths λ₁ and λ₂ in FIG. 1. The power of the input signalsis −20 dBm/ch, and the wavelengths are arranged at an even space between1540 nm and 1560 nm. The amplifier fiber 2 was a dispersion compensatingfiber having a length of about 6 km, and powers of the pumping lightswere adjusted to compensate the loss of the dispersion compensatingfiber while keeping deviation between channels within 0.5 dB.

Second Embodiment of Raman Amplifier

FIG. 2 shows a second embodiment of a Raman amplifier according to thepresent invention, in which the pumping light from the pumping means 1is propagated toward the same direction of the optical signal in theamplifier fiber 2. More specifically, the WDM coupler 13 is provided ata input end of the amplifier fiber 2, and the pumping light from thepumping means 1 is transmitted, through the WDM coupler 13, from theinput end to output end of the amplifier fiber 2. In this arrangement,since amplification is effected before attenuation of signal, the noiseproperty of the optical signal is superior to that in the firstembodiment. Further, it was found that the gain is smaller in comparisonwith the first embodiment.

Third Embodiment of Raman Amplifier

FIG. 3 shows a third embodiment of a Raman amplifier according to thepresent invention, in which pumping lights from an pumping means 1 arepropagated in two directions through the amplifier fiber 2. Morespecifically, WDM couplers 13 are provided at input and output ends ofthe amplifier fiber 2, respectively, and the pumping lights from twogroups of the pumping means 1 are coupled to the amplifier fiber 2through the respective WDM couplers 13 so that the pumping lightlaunched into the input side WDM coupler 13 is propagated toward theoutput end of the amplifier fiber 2 and the pumping light launched intothe output side WDM coupler 13 is propagated toward the input end of theamplifier fiber 2.

Central wavelengths of semiconductor lasers 3 ₁, 3 ₂ included in thefirst group A of the pumping means 1 and central wavelengths ofsemiconductor lasers 3 ₅, 3 ₆ included in the second group B are thesame, and central wavelengths of semiconductor lasers 3 ₃, 3 ₄ includedin the first group A and central wavelengths of semiconductor lasers 3₇, 3 ₈ included in the second group B are the same. Further, fibergratings 5 ₁ to 5 ₈ are matched with the central wavelength of thesemiconductor lasers 3 to which the fiber gratings are connected,respectively.

Fourth Embodiment of Raman Amplifier

In the embodiment shown in FIG. 3, when it is assumed that the centralwavelengths of the semiconductor lasers 3 ₁, 3 ₂ included in the firstgroup A are λ₁, central wavelengths of the semiconductor lasers 3 ₃, 3 ₄included in the first group A are λ₃, central wavelengths of thesemiconductor lasers 3 ₅, 3 ₆ included in the second group B are λ₂, andcentral wavelengths of the semiconductor lasers 3 ₇, 3 ₈ included in thesecond group B are λ₄, the wavelengths λ₂, λ₃, λ₄ may be adjacentwavelengths. Also in this case, the interval between the centralwavelengths is greater than 6 nm and smaller than 35, and the differencebetween the maximum central wavelength λ₄ and the minimum centralwavelength λ₁ is smaller than 100 nm. With this arrangement, thewavelength interval of the pumping lights combined in the same group canhave play or margin and performance required for the WDM couplers 13 canbe loosened.

Fifth Embodiment of Raman Amplifier

FIG. 40 shows a fifth embodiment of a Raman amplifier according to thepresent invention. In this embodiment, appropriate Raman amplifiers areselected among the Raman amplifiers 9 described in the above-mentionedembodiments and the selected Raman amplifiers are connected together ina multi-stage fashion. By properly selecting the Raman amplifiers 9having different characteristics in accordance with the desiredamplifying property and noise property, a property which could not beachieved by a single Raman amplifier can be obtained.

In the above-mentioned embodiments, the output light power controllingmeans 4 may be designed as shown in FIG. 4 or FIG. 5. In an arrangementshown in FIG. 4, a monitor signal detecting and LD control signalgenerating circuit 15 comprising a wavelength demultiplexer 18,optical/electrical converting means 19 such as photo-diodes and an LDcontrol circuit 20 is connected to a monitor light branching coupler 14as shown in FIG. 1, 2, or 3. The wavelength demultiplexer 18 serves todemultiplex the output light branched by the monitor light branchingcoupler 14 into a plurality of wavelength lights. In this case, lightsnear the maximum amplification wavelengths (each of which is awavelength obtained by adding 100 nm to the pumping light wavelength) ofthe respective pumping lights are demultiplexd (more specifically, whenthe pumping wavelengths are 1430 nm and 1460 nm, wavelength lights near1530 nm and 1560 nm are demultiplexd). Each of the optical/electricalconverting means 19 serves to convert the received wavelength light intoan electrical signal, so that output voltage is varied with magnitude ofthe light receiving level. The LD control circuit 20 serves to changethe drive currents for the semiconductor lasers 3 in accordance with theoutput voltages from the optical/electrical converting means 19, and, bycalculating and processing the output voltage from theoptical/electrical converting means 19, the semiconductor lasers 3 arecontrolled so that light powers of the wavelength lights are ordered oraligned. That is to say, the output light power controlling means 4 actsto eliminate the wavelength dependency of the Raman gain thereby toflatten the gain.

In an arrangement shown in FIG. 5, a monitor signal detecting and LDcontrol signal generating circuit 15 comprising a power splitter 21,band pass filters 22, optical/electrical converting means 19 such asphoto-diodes and an LD control circuit 20 is connected to a monitorlight branching coupler 14. The power splitter 21 serves to branch theoutput light branched by the monitor light branching coupler 14 intolights having the same number as the pumping lights. The band passfilters 22 have different permeable central wavelengths, and, in thiscase, permit to pass lights near the maximum amplification wavelengths(each of which is a wavelength obtained by adding 100 nm to the pumpinglight wavelength) of the respective pumping lights (more specifically,when the pumping wavelengths are 1430 nm and 1460 nm, pass of wavelengthlights near 1530 nm and 1560 nm are permitted). Each of theoptical/electrical converting means 19 serves to convert the receivedwavelength light into an electrical signal, so that output voltage isvaried with magnitude of the light receiving level. The LD controlcircuit 20 serves to change the drive currents for the semiconductorlasers 3 in accordance with the output voltages from theoptical/electrical converting means 19, and, by calculating andprocessing the output voltage from the optical/electrical convertingmeans 19, the semiconductor lasers 3 are controlled so that light powersof the wavelength lights are ordered or aligned. That is to say, theoutput light power controlling means 4 acts to eliminate the wavelengthdependency of the Raman gain thereby to flatten the gain. Although FIGS.4 and 5 show arrangements for controlling the pumping means 1 bymonitoring the output light as shown in FIG. 27, as shown in FIG. 26, anarrangement for controlling the pumping means 1 by monitoring the inputlight can be utilized, or, as shown in FIG. 28, an arrangement forcontrolling the pumping means 1 by monitoring both the input light andthe output light can be utilized.

In the Raman amplifiers having the above-mentioned constructions, inplace of the fact the pumping lights are combined by the polarizationwave composing coupler 6, as shown in FIGS. 6A and 6B, a polarizationplane rotating means 7 for rotating polarization plane of the pumpinglight by 90 degrees may be provided, so that the plural pumping lightsgenerated by the pumping means 1 and pumping lights having polarizationplanes perpendicular to those of the former pumping lights aresimultaneously pumped in the amplifier fiber 2. FIG. 6A shows anarrangement in which a Faraday rotator 3 ₁ and a total reflecting mirror3 ₂ are provided at an end of the amplifier fiber 2 so that thepolarization plane of the pumping light propagated to the amplifierfiber 2 is rotated by 90 degrees to be returned to the amplifier fiber 2again. In FIG. 6, a means for picking up the optical signal propagatedto and Raman-amplified in the amplifier fiber 2 from the fiber 2 is notshown. FIG. 6B shows an arrangement in which a PBS 33 and a polarizationmaintaining fiber 34 are provided at an end of the amplifier fiber 2 sothat the polarization plane of the pumping light outputted from the endof the amplifier fiber 2 is rotated by 90 degrees by the polarizationmaintaining fiber 34 connected to twist its main axis by 90 degrees, tobe inputted to the end of the amplifier fiber 2 again through the PBS33.

First Embodiment of Optical Repeater

FIG. 7 shows a first embodiment of an optical repeater constructed byusing the Raman amplifier according to the present invention. In thisexample, the optical repeater is inserted into an optical fibertransmission line 8 to compensate loss in the optical fiber transmissionline 8. In this optical repeater, a rare earth doped fiber amplifier(referred to as “EDFA” hereinafter) 10 is connected to a rear stage ofthe Raman amplifier 9 as shown in FIG. 1, 2 or 3 so that the opticalsignal transmitted to the optical fiber transmission line 8 is inputtedto the Raman amplifier 9 to be amplified and then is inputted to theEDFA 10 to be further amplified and then is outputted to the opticalfiber transmission line 8. The gain of the repeater may be adjusted bythe Raman amplifier 9 or by the EDFA 10 or by both amplifiers so long asthe loss of the optical fiber transmission line 8 can be compensated intotal. Further, by properly combining a difference between thewavelength dependency of gain of the EDFA 10 and the wavelengthdependency of the Raman amplifier 9, the wavelength dependency of gainof the EDFA 10 can be reduced by the wavelength dependency of the Ramanamplifier 9.

Second Embodiment of Optical Repeater

FIG. 8 shows a second embodiment of an optical repeater constructed byusing the Raman amplifier according to the present invention. Accordingto this embodiment, in the optical repeater shown in FIG. 7, anadditional EDFA 10 is also provided at a front stage of the Ramanamplifier 9. Incidentally, the EDFA 10 may be provided only at the frontstage of the Raman amplifier 9.

Third Embodiment of Optical Repeater

FIG. 9 shows a third embodiment of an optical repeater constructed byusing the Raman amplifier according to the present invention. Accordingto this embodiment, there is provided a Raman amplifier 9 in which adispersion compensating fiber (DCF) is used as the amplifier fiber 2between two EDFAs 10. Between the Raman amplifier 9 and the EDFA 10 atthe rear stage thereof, there are provided a branching coupler 23 forbranching the output light from the Raman amplifier 9, and a monitorsignal detecting and LD control signal generating circuit 24 formonitoring the branched light and for controlling the gain of the Ramanamplifier 9. The monitor signal detecting and LD control signalgenerating circuit 24 is a control circuit capable of keeping the outputpower of the Raman amplifier 9 to a predetermined value. Incidentally,when the Raman amplifier 9 itself has the controlling means 4 as shownin FIG. 4 or FIG. 5, the power of the output light is controlled tobecome the predetermined value, and, at the same time, the power of thepumping light is controlled so that level deviation between pluraloutput signals become small.

In the optical repeater shown in FIG. 9, the output light level of theRaman amplifier 9, i.e., input level to the second EDFA 10 is alwayskept constant without being influenced by the loss of the DCF and theoutput level of the first EDFA 10. This ensures that, when the output ofthe repeater is defined, the gain of the second EDFA 10 is keptconstant. In this way, deterioration of gain flatness of the second EDFA10 due to fluctuation in loss of the DCF is avoided, Further, when thefirst EDFA 10 is controlled so that the gain becomes constant, thefluctuation of input to the repeater is compensated by the varying ofgain of the Raman amplifier 9, Namely, the adjustment is effected onlyon the basis of the gain of the Raman amplifier 9, with the result thatthe deterioration of gain flatness due to fluctuation in gain of theEDFA 10 can be avoided completely.

Fourth Embodiment of Optical Repeater

FIG. 10 shows an example that, in the embodiment shown in FIG. 9, acontrol means for adjusting the gain of the Raman amplifier 9 bymonitoring the light level is also provided between the first EDFA 10and the Raman amplifier 9. By using such control means, the pumpinglight can be controlled to keep the difference between the input andoutput levels of the Raman amplifier 9 constant, with the result thatonly the dispersion of the loss of the DCF can be compensated.

Fifth Embodiment of Optical Repeater

FIG. 11 shows an example that, in the above-mentioned embodiment, thegain flattening monitor mechanism is shifted to the output end of therepeater and is used as a monitor for flattening the gain of the entirerepeater. In this case, the first EDFA 10 and the second EDFA 10 mayperform the gain constant control or the output constant control. Thepowers of the pumping lights are independently controlled to reduce thelevel deviation between the output signals at the output of therepeater.

Sixth Embodiment of Optical Repeater

In an optical repeater according to a sixth embodiment of the presentinvention, a dispersion compensating fiber is used as the amplifierfiber 2 of the Raman amplifier shown in FIG. 1, 2 or 3 to compensatedispersion of wavelength of the optical fiber transmission line 8, sothat the losses in the optical fiber transmission line 8 and theamplifier fiber 2 are compensated partially or totally.

Seventh Embodiment of Optical Repeater

In the above-mentioned embodiment of the optical repeater, a Ramanamplifier 9 using an pumping means 1 shown in FIG. 41, 42, 43 or 44 maybe constituted.

Eighth Embodiment of Optical Repeater

As shown in FIGS. 29 to 32, WDM couplers 13 are inserted on the way ofthe amplifier fiber 2 of the Raman amplifier 9 so that residual pumpinglight from the pumping means 1 propagated to the amplifier fiber 2 isinputted to the optical fiber transmission line 8 through a WDM coupler27 provided in the optical fiber transmission line 8 at the input oroutput side of the Raman amplifier 9, thereby also generating Raman gainin the optical fiber transmission line 8. Incidentally, in FIGS. 29 to32, the reference numeral 26 denotes an isolator.

Ninth Embodiment of Optical Repeater

As shown in FIGS. 33 to 36, when the optical repeater comprises theRaman amplifier 9 and the EDFA 10, WDM couplers 13 are inserted on theway of the amplifier fiber 2 of the Raman amplifier 9 so that residualpumping light from the pumping means 1 propagated to the amplifier fiber2 is inputted to the EDFA 10 to be used as pumping light/auxiliarypumping light for the EDFA 10. Incidentally, in FIGS. 33 to 36, thereference numeral 26 denotes an isolator.

First Embodiment of Raman Amplifying Method

An embodiment of a Raman amplifying method according to the presentinvention will now be fully explained with reference to FIGS. 49 to 52.In this embodiment, a dispersion compensating fiber (DCF) having highnon-linearity is used in a Raman amplifying medium 50 shown in FIG. 49and pumping light generated from an pumping light source 51 is inputtedand transmitted to the Raman amplifying medium through a wave combiner52. In this case, as shown in FIG. 50, as the pumping light source 50, a4ch-WDMLD unit comprising four pumping light sources (semiconductorlasers), fiber Bragg gratings (FBG), polarization beam combiner (PBC)and a WDM is used. The semiconductor lasers shown in FIG. 50 generatepumping lights having different central wavelengths (more specifically,generate pumping lights having central wavelengths of 1435 nm, 1450 nm,1465 nm and 1480 nm, respectively). These pumping lights give Raman gainto an optical signal transmitted from the DCF, thereby amplifying theoptical signal. In this case, each of the pumping lights has gain peakat frequency shorter than its frequency by about 13 THz (i.e.,wavelength greater by about 100 nm).

When the optical signal having wavelength of 1500 nm to 1600 nm ispropagated to the DCF having a length of 6 km from one end thereof andthe pumping lights having wavelengths of 1400 nm, 1420 nm, 1440 nm, 1460nm, and 1480 nm are inputted to the DCF thereby to Raman-amplify theoptical signal, the input optical signal and an output optical signaloutputted from the other end of the DCF (Raman-amplified optical signal)were checked to evaluate total loss of the wavelength and DCF. Thefollowing Table 3 shows a relationship between the wavelength and DCFtotal loss. From this, it is apparent that there is wavelengthdependency: TABLE 3 wavelength (nm) unit loss (dB/km) total loss (dB)1400 6.76 40.56 1420 3.28 19.68 1440 1.74 10.44 1460 1.16 6.96 1480 0.855.10 1500 0.69 4.14 1520 0.62 3.72 1540 0.57 3.42 1560 0.55 3.30 15800.54 3.24 1600 0.59 3.54

Here, when it is considered that effect total loss is given by addingthe loss of the semiconductor laser to the loss of the optical signalamplified by the semiconductor laser at the side of the longerwavelength greater by about 100 nm, a relationship between thewavelength and the effect total loss becomes as shown in the followingTABLE 4 wavelength (nm) effect total loss (dB) 1400 40.56 + 4.14 44.71420 19.68 + 3.72 23.4 1440 10.44 + 3.42 13.86 1460  6.96 + 3.30 10.261480  5.10 + 3.24 8.34

Since there is substantially no wavelength dependency of the Ramanamplification itself, when it is regarded that amplifying efficiency foreach wavelength is influenced by the effect total loss, by adding theeffect total loss to the outputs of the semiconductor lasers requiredfor desired amplifying properties, the wavelengths can beRaman-amplified substantially uniformly, and the wavelength dependencyof the gain can be eliminated. Thus, in this embodiment, the shorter thecentral wavelength of the pumping light the higher the light power.

Second Embodiment of Raman Amplifying Method

In a Raman amplifying method according to a second embodiment of thepresent invention, in order to Raman-amplify substantially uniformly theoptical signals having wavelengths of 1500 nm to 1600 nm transmitted bythe DCF, among two or more pumping lights incident on the DCF, lightpowers of pumping lights having wavelengths smaller than center (middle)between the shortest central wavelength and the longest centralwavelength are increased. More specifically, central wavelengths of thepumping lights generated by the pumping light source 51 shown in FIG. 50and incident on the DCF are selected to 1435 nm, 1450 nm, 1465 nm and1480 nm and the light powers thereof are selected as follows. That is tosay, the light powers of the pumping lights having the wavelengths of1435 nm and 1450 nm which are smaller than the center (1457 nm) betweenthe shortest central wavelength (1435 nm) and the longest centralwavelength (1480 nm) among four pumping lights incident on the DCF areincreased, as follows: Light power of 1435 nm 563 mW Light power of 1450nm 311 mW Light power of 1465 nm 122 mW Light power of 1480 nm 244 mW

As a result, the gain profile of the optical signal having wavelength of1500 nm to 1600 nm (after Raman-amplified) transmitted by the DCFprovides gain of about 11 dB from 1540 nm to 1590 nm, with the resultthat the flatness becomes 1 dB, as shown in FIG. 51. That is to say, thewavelength lights transmitted by the DCF can be Raman-amplifiedsubstantially uniformly.

Incidentally, when the central wavelengths of the pumping lightsgenerated by the pumping light source 51 and incident on the DCF areselected to 1435 nm, 1450 nm, 1465 nm and 1480 nm and the light powersthereof are selected to 563 mW uniformly, the gain profile of theoptical signal having wavelength of 1500 nm to 1600 nm (afterRaman-amplified) transmitted by the DCF becomes as shown in FIG. 52.That is to say, although gain of about 24 dB can be obtained near thewavelength of 1580 nm, wide band gain flatness cannot be achieved (lossspectrum of the fiber is turned over).

Third Embodiment of Raman Amplifying Method

FIG. 53 shows a third embodiment of a Raman amplifying method accordingto the present invention. In the Raman amplifying method shown in FIG.53, pumping lights are combined by a wave combiner utilizing a principleof a Mach-Zehnder interferometer. The wavelengths of the pumping lightswhich can be combined becomes equidistant. In this embodiment, amongwavelengths which can be combined, several wavelengths are not used, andthe wavelength number at the short wavelength side of the pumping lightband is greater than the wavelength number at the long wavelength side.In this arrangement, when the power of all of the pumping lights havingvarious wavelengths are the same, the total power of the pumping lightsat the short wavelength side becomes greater than the total power of thepumping lights at the long wavelength side. This provides substantiallythe same effect, similar to the second embodiment, as the effectobtained by setting the power at the short wavelength side to be greaterthan the power at the long wavelength side under the condition that thepumping lights are arranged equidistantly. Accordingly, by setting asshown in FIG. 53, the gain profile can be flattened without creatinggreat difference between the powers of the pumping lights. This meansthat the total power of the pumping lights in which gain profile in apredetermined band can be flattened can be increased with determiningthe upper limit of the output power from one pumping light and thereforethat the gain of the amplifier can be increased.

Sixth Embodiment of Raman Amplifier

In an embodiment described hereinbelow, the shortest wavelengths of thepumping lights to be used are selected to 1420.8 nm (211 THz). Thereason is that the wavelength greater than 1530 nm (frequency belowabout 196 THz) which has been utilized in the present WDM systems isconsidered to be used as an amplification band. Accordingly, when awavelength greater than 1580 nm (frequency below about 190 THz) which isreferred to as an L-band is supposed as the amplification band, sincethe pumping band may be shifted by 6 THz, the shortest wavelength may beselected to 1462.4 nm (205 THz). Regarding the other amplificationbands, the shortest wavelengths can be determined in the similar manner.

FIG. 60 is a six embodiment of a Raman amplifier according to thepresent invention which corresponds to Claim 32. Frequency of a firstchannel is 211 THz (wavelength of 1420.8 nm) and frequencies of secondto fifth channels are from 210 THz (wavelength of 1427.6 nm) to 207 THz(wavelength of 1448.3 nm) and are arranged side by side with an intervalof 1 THz. By combining this with an pumping light (frequency of 205 THz,wavelength of 1462. 4 nm) having a wavelength spaced apart from thefifth channel by 2 THz toward the long wavelength side, the pumpingmeans is formed. Accordingly, a distance or interval between theadjacent pumping wavelengths is within a range from 6 nm to 35 nm, andthe number of pumping light sources having center wavelengths at theshort wavelength side with respect to the center between the shortestcenter wavelength and the longest center wavelength of each pumpinglight becomes greater than the number of pumping light sources havingcenter wavelengths at the long wavelength side. The pumping light ofeach channel utilizes an pumping light obtained by combining lights fromsemiconductor lasers of Fabry-Perot type (wavelengths of which arestabilized by fiber Bragg gratings (FBG)) by means of a polarizationbeam combiner (PBC). Polarization wave composing is effected so as toincrease the pumping power of each wavelength and to reduce thepolarization dependency of the Raman gain. When the pumping powerobtained by output from the single laser is adequate, the laser outputmay be connected to the wavelength combiner after depolarizing.

FIG. 61 shows Raman gain profiles when the pumping light sources shownin FIG. 60 are used. A curve A represents total gain, a curve Brepresents the sum of gains of the pumping lights of the first to fifthchannels, a curve C represents a gain of the sixth channel, and thinlines represent gains of pumping wavelengths of the first to fifthchannels. By multiplexing the pumping lights at the short wavelengthside with interval 1 THz, a smooth curve extending rightwardly anddownwardly can be formed, and, by adding this to a gain curve extendingrightwardly and upwardly due to the pumping lights at the longwavelength side, the total Raman gain is flattened. According to FIG.61, the gain obtained from the pumping lights at the short wavelengthside may be relatively small, but, since there are wavelength dependencyof loss of the pumping lights and Raman effect between the pumpinglights, the actual incident power at the short wavelength side must begreater than that at the long wavelength side. From FIG. 61, it can beseen that a projection of certain gain curve and a recess of anothergain curve cancel mutually by using the interval of 1 THz. FIG. 62 is anenlarged view of the total gain. A property in which the peak gain is 10dB, the gain band extends from about 196 THz (wavelength of 1529.6 nm)to about 193 THz (wavelength of 1553.3 nm) and the gain deviation isabout 0.1 dB is achieved.

FIG. 63 shows gain profiles when the wavelength of the sixth channel isa wavelength (frequency of 204.5 THz; wavelength of 1465.5 nm) spacedapart from the wavelength of the fifth channel by 2.5 THz toward thelong wavelength side in FIG. 60. Similar to FIG. 61, a curve Arepresents total gain, a curve B represents the sum of gains of thepumping lights of the first to fifth channels, a curve C represents again of the sixth channel, and thin lines represent gains of pumpingwavelengths of the first to fifth channels. Also in this case, by addinga gain curve extending rightwardly and downwardly due to the pumpinglights at the short wavelength side to a gain curve extendingrightwardly and upwardly due the pumping lights at the long wavelengthside, the total Raman gain is flattened. According to FIG. 63, the gainobtained from the pumping lights at the short wavelength side may berelatively small, but, since there are wavelength dependency of loss ofthe pumping lights and Raman effect between the pumping lights, theactual incident power at the short wavelength side must be greater thanthat at the long wavelength side. FIG. 64 is an enlarged view of thetotal gain. A property in which the peak gain is 10 dB, the gain bandextends from about 196 THz (wavelength of 1529.6 nm) to about 192 THz(wavelength of 1561.4 nm) and the gain deviation is about 0.1 dB isachieved. The gain band is wider than that in FIG. 62, and recess of thegain at a middle portion of the band is slightly greater. The reason isthat the interval or distance between the fifth channel and the sixthchannel becomes wider.

Seventh Embodiment of Raman Amplifier

FIG. 65 shows a seventh embodiment of a Raman amplifier according to thepresent invention which corresponds to Claims 32 and 33. Frequency of afirst channel is 211 THz (wavelength of 1420.8 nm) and frequencies ofsecond to eighth channels are from 210 THz (wavelength of 1427.6 nm) to204 THz (wavelength of 1469.6 nm) and are arranged side by side with aninterval of 1 THz. The total number of channels is eight, and pumpinglight sources are constituted by using six wavelengths except for sixthand seventh channels. Accordingly, a distance or interval between theadjacent pumping wavelengths is within a range from 6 nm to 35 nm, andthe number of pumping light sources having center wavelengths at theshort wavelength side with respect to the center between the shortestcenter wavelength and the longest center wavelength of each pumpinglight becomes greater than the number of pumping light sources havingcenter wavelengths at the long wavelength side. The pumping lights ofthe channels are selected on demand, as described in connection with thesixth embodiment. FIG. 66 shows Raman gain profiles when the pumpinglight sources shown in FIG. 65 are used. A curve A represents totalgain, a curve B represents the sum of gains of the pumping lights of thefirst to fifth channels, a curve C represents a gain of the eighthchannel, and thin lines represent gains of pumping wavelengths of thefirst to fifth channels. Also in this case, by adding a gain curveextending rightwardly and downwardly due to the pumping lights at theshort wavelength side to a gain curve extending rightwardly and upwardlydue to the pumping lights at the long wavelength side, the total Ramangain is flattened. According to FIG. 66, the gain obtained from thepumping lights at the short wavelength side may be relatively small,but, since there are wavelength dependency of loss of the pumping lightsand Raman effect between the pumping lights, the actual incident powerat the short wavelength side must be greater than that at the longwavelength side. FIG. 67 is an enlarged view of the total gain. Aproperty in which the peak gain is 10 dB, the gain band extends fromabout 196 THz (wavelength of 1529.6 nm) to about 191 THz (wavelength of1569.6 nm) and the gain deviation is about 0.1 dB is achieved. Incomparison with FIG. 62 and FIG. 64, the gain band becomes furtherwider. The reason is that the longest pumping wavelength is set tofurther longer.

Eighth Embodiment of Raman Amplifier

FIG. 68 shows an eighth embodiment of a Raman amplifier according to thepresent invention which corresponds to Claims 32 and 34. Frequency of afirst channel is 211 THz (wavelength of 1420.8 nm) and frequencies ofsecond to eighth channels are from 210 THz (wavelength of 1427.6 nm) to204 THz (wavelength of 1469.6 nm) and are arranged side by side with aninterval of 1 THz. The total number of channels is eight, and pumpinglight sources are constituted by using six wavelengths except for fifthand sixth channels. Accordingly, a distance or interval between theadjacent pumping wavelengths is within a range from 6 nm to 35 nm, andthe number of pumping light sources having center wavelengths at theshort wavelength side with respect to the center between the shortestcenter wavelength and the longest center wavelength of each pumpinglight becomes greater than the number of pumping light sources havingcenter wavelengths at the long wavelength side. The pumping lights ofthe channels are selected on demand, as described in connection with thesixth embodiment. FIG. 69 shows Raman gain profiles when the pumpinglight sources shown in FIG. 68 are used. A curve A represents totalgain, a curve B represents the sum of gains of the pumping lights of thefirst to fourth channels, a curve C represents the sum of gains of theseventh and eighth channels, and thin lines represent gains of pumpingwavelengths. Also in this case, by adding a gain curve extendingrightwardly and downwardly due to the pumping lights at the shortwavelength side to a gain curve extending rightwardly and upwardly dueto the pumping lights at the long wavelength side, the total Raman gainis flattened. According to FIG. 69, the gain obtained from the pumpinglights at the short wavelength side may be relatively small, but, sincethere are wavelength dependency of loss of the pumping lights and Ramaneffect between the pumping lights, the actual incident power at theshort wavelength side must be greater than that at the long wavelengthside. FIG. 70 is an enlarged view of the total gain. A property in whichthe peak gain is 10 dB, the gain band extends from about 196 THz(wavelength of 1529.6 nm) to about 191 THz (wavelength of 1569.6 nm) andthe gain deviation is about 0.1 dB is achieved. Here, it is noticeablethat magnitude of the gains of the pumping wavelengths in the seventhembodiment differs from that in the eighth embodiment. Namely, there isa channel of about 8 dB at the maximum in FIG. 66; whereas, about 5 dBis maximum in FIG. 69. The reason is that the gain at the longwavelength side shown by the curve C is formed by the gain of onechannel in the seventh embodiment, whereas, the gain at the longwavelength side is formed by the sum of the gains of two channels in theeighth embodiment. This means that a maximum value of the pumping lightpower required for one wave can be reduced, which is very effective inthe viewpoint of practical use.

FIGS. 71 to 74 show gain profiles when eleven channels are used amongthirteen channels arranged side by side at an interval of 1 THz from 211THz (wavelength of 1420.8 nm) to 199 THz (wavelength of 1506.5 nm). FIG.71 shows the gain profiles obtained by an arrangement corresponding toClaim 33 and by using the pumping lights other than 201 THz and 200 THz.Accordingly, a distance or interval between the adjacent pumpingwavelengths is within a range from 6 nm to 35 nm, and the number ofpumping light sources having center wavelengths at the short wavelengthside with respect to the center between the shortest center wavelengthand the longest center wavelength of each pumping light becomes greaterthan the number of pumping light sources having center wavelengths atthe long wavelength side. A curve A represents total gain, a curve Brepresents the sum of gains of the pumping lights of the first to tenthchannels, a curve C represents the gain of the thirteenth channel, andthin lines represent gains of pumping wavelengths of first to tenthchannels. Also in this case, by adding a gain curve extendingrightwardly and downwardly due to the pumping lights at the shortwavelength side to a gain curve extending rightwardly and upwardly dueto the pumping lights at the long wavelength side, the total Raman gainis flattened. According to FIG. 71, the gain obtained from the pumpinglights at the short wavelength side may be relatively small, but, sincethere are wavelength dependency of loss of the pumping lights and Ramaneffect between the pumping lights, the actual incident power at theshort wavelength side must be greater than that at the long wavelengthside. FIG. 72 is an enlarged view of the total gain. A property in whichthe peak gain is 10 dB, the gain band extends from about 196 THz(wavelength of 1529.6 nm) to about 186 THz (wavelength of 1611.8 nm) andthe gain deviation is about 0.1 dB is achieved.

FIG. 73 shows the gain profiles obtained by an arrangement correspondingto Claim 34 and by using the pumping lights other than 202 THz and 201THz. Accordingly, a distance or interval between the adjacent pumpingwavelengths is within a range from 6 nm to 35 nm, and the number ofpumping light sources having center wavelengths at the short wavelengthside with respect to the center between the shortest center wavelengthand the longest center wavelength of each pumping light becomes greaterthan the number of pumping light sources having center wavelengths atthe long wavelength side. A curve A represents total gain, a curve Brepresents the sum of gains of the pumping lights of the first to ninthchannels, a curve C represents the sum of the gains of the twelfth andthirteenth channels, and thin lines represent gains of pumpingwavelengths. Also in this case, by adding a gain curve extendingrightwardly and downwardly due to the pumping lights at the shortwavelength side to a gain curve extending rightwardly and upwardly dueto the pumping lights at the long wavelength side, the total Raman gainis flattened. According to FIG. 73, the gain obtained from the pumpinglights at the short wavelength side may be relatively small, but, sincethere are wavelength dependency of loss of the pumping lights and Ramaneffect between the pumping lights, the actual incident power at theshort wavelength side must be greater than that at the long wavelengthside. FIG. 74 is an enlarged view of the total gain. A property in whichthe peak gain is 10 dB, the gain band extends from about 196 THz(wavelength of 1529.6 nm) to about 186 THz (wavelength of 1611.8 nm) andthe gain deviation is about 0.1 dB is achieved. Further, as apparentfrom the comparison of FIG. 71 with FIG. 73, since the gain at the longwavelength side shown by the curve C is formed by the gain of onechannel in the seventh embodiment, whereas, the gain at the longwavelength side is formed by the sum of the gains of two channels in theeighth embodiment, a maximum value of the gain required for one wave issmaller in FIG. 73 than in FIG. 71. This means that a maximum value ofthe pumping light power required for one wave can be reduced, which isvery effective in the viewpoint of practical use.

Ninth Embodiment of Raman Amplifier

FIGS. 54A and 54B show Raman gain profiles when the intervals betweenthe pumping lights are 4.5 THz and 5 THz, respectively and DSF is usedas the amplifier fiber. As apparent from FIGS. 54A and 54B, as theinterval between the pumping lights is increased, the recess or valleyof the gain becomes deeper, and, thus, the gain deviation becomesgreater. Incidentally, in FIG. 54A, values shown in the following Table1 are used as frequencies (wavelengths) of the pumping lights, and, inFIG. 54B, values shown in the following Table 2 are used as frequencies(wavelengths) of the pumping lights. In such cases, the interval of 4.5THz between the pumping lights corresponds to 33 nm, and 5 THzcorresponds to 36.6 nm. That is to say, from these examples, it can beseen that, if the interval between the pumping lights is greater than 35nm, the gain flatness is worsened. TABLE 1 Pumping frequency pumpingwavelength wavelength interval THz nm nm 204.5 1466.0 33.0 200.0 1499.0

TABLE 2 Pumping frequency pumping wavelength wavelength interval THz nmnm 205.0 1462.4 36.6 200.0 1499.0

FIGS. 55 shows gain profiles when the interval between the pumpinglights is 4.5 THz, and three wavelengths are used. From FIG. 55, when athird pumping wavelength is added, it can be seen that, if the intervalbetween the pumping lights is 4.5 THz, the recess of the gain becomesdeeper. FIG. 56 shows gain profiles when the intervals between thepumping lights are 2.5 THz and 4.5 THz and three wavelengths are used.In comparison with FIG. 55, the recess of the gain is shallower. Sincethe frequency interval of 2.5 THz used in this case corresponds to about18 nm between the wavelengths, also in this case, it can be said thatthe interval between the adjacent pumping wavelengths is within a rangefrom 6 nm to 35 nm.

INDUSTRIAL AVAILABILITY

In the Raman amplifier according to the present invention, by selectingthe wavelengths of the pumping light sources so that the intervalbetween the central wavelengths becomes greater than 6 nm and smallerthan 35 nm and the difference between the maximum central wavelength andthe minimum central wavelength becomes within 100 nm, there can beprovided an optical amplifier in which the wavelength dependency of gainis reduced to the extent that the gain flattening filter is not requiredand in which, if the gain is changed, the flatness can be maintained.Further, this amplifier can be applied to an optical repeater forcompensating loss of a transmission line and wavelength dispersion. In arepeater constituted by the combination of the amplifier and EDFA,fluctuation of the EDFA due to fluctuation of input of the repeaterand/or fluctuation in loss of DCF can be suppressed to avoiddeterioration of gain flatness and the repeater can be applied tovarious systems.

In the Raman amplifying method according to the present invention, sincethe shorter center wavelength of the pumping light among two or morepumping lights incident on the DCF the greater the power, or, since thepower of the pumping light at the short wavelength side with respect tothe center between the shortest central wavelength and the longestcentral wavelength among two or more pumping lights incident on the DCFis increased, in any cases, even when an optical fiber having highnon-linearity is used, wavelength multiplexing lights of about 1500 nmto about 1600 nm can be amplified with substantially the same gain. Inother words, by using the optical fiber having high non-linearity, therequired gain can be obtained even with a short optical fiber. Further,since the optical fiber can be shortened, a Raman amplifier which can beunitized can be provided.

As described in connection with the prior art, the wavelength combinerof Mach-Zehnder interferometer type is very useful for multiplexing thepumping lights efficiently. One of reasons why the pumping lights havingthe equal frequency interval are used is that such a wave combiner canbe used. FIGS. 57 to 59 show performance of Raman gain curves when theinterval between the pumping lights is equidistant. FIG. 57 shows acondition that adjustment is effected to bring the peak gain to 10 dBunder a condition that the gains of the pumping lights are the same.From FIG. 57, it can be seen that the smaller the interval between thepumping lights the smaller the unevenness of the gains. FIG. 58 shows anexample that the gains of the pumping lights are adjusted to flatten thegains. Also in this case, similar to FIG. 57, the interval between thepumping lights the smaller the unevenness of the gains. Further, it canbe seen that undulation of the gain curves in FIG. 57 determines themaximum gain deviation in FIG. 58. Thus, in order to keep the gaindeviation to about 0.1 dB, it is said that, although the interval of 2THz between the pumping lights is too great, 1 THz is adequate.

FIG. 59 shows performance when the interval between the pumping lightsis 1 THz and the multiplexing number is changed. As can be seen from a 1ch pumping gain curve, when a silica-based fiber is used, although asmooth curve without unevenness is presented at the short wavelengthside with respect to the gain peak, there are three noticeable localpeaks at the long wavelength side, and such unevenness becomes a factorfor determining limit of flattening. Such unevenness is reduced as thenumber of multiplexing is increased. For example, observing the 1 chpumping gain curve, although there is protrusion of about 1 dB near 187THz, as the number of multiplexing is increased, the protrusion isreduced. The reason is that, since the peak gains are set to be thesame, the gain per one pumping wavelength or the local peak itself isreduced as the number of pumping wavelength is increased, and that theslightly and equidistantly shifted unevenness having the sameconfiguration are added. That is to say, by adding the protrusion of thegain curve of a certain pumping wavelength to the recess of the gaincurve of another pumping wavelength, the unevenness is reduced totally.A value of “about 1 THz” defined in Claims 32 to 34 is based on thisprinciple and is based on the fact that, in the 1 ch pumping gain curveshown in FIG. 59, frequency difference between the protrusion near 187THz and the adjacent recess near 188 THz is about 1 THz. Accordingly,depending upon a fiber used, the 1 ch pumping gain curve may be slightlydifferentiated, and, thus, the value of “about 1 THz” described inClaims 32 to 34 may be changed. In any cases, in order to reduce thegain deviation, it is required that the unevenness (projections andrecesses) of the gain curves to be added or combined to be cancelled.

Since the limit of the gain deviation is determined by undulation and/orunevenness of the gain curves to be overlapped, it is considered thatthe gain profile having good flatness and small gain deviation can beobtained by combining gain curves having less unevenness. Accordingly,this can be achieved by combining the gain curve obtained bymultiplexing the pumping lights at interval of 1 THz with the gain curveof the pumping light at the long wavelength side with respect to saidpumping wavelength. In this case, it is desirable that the peaks of twogain curves are moderately spaced apart from each other in the viewpointof the widening of the band.

1. An optical repeater, comprising: an optical fiber configured to be aRaman amplifying medium and having an input end and an output end andconfigured to receive a wavelength division multiplex, WDM, opticalsignal having a signal bandwidth of at least 20 nm; an output opticaltransmission fiber connected to the output end of the optical fiber; aninput optical transmission fiber connected to the input end of theoptical fiber; a WDM coupler configured to optically couple light to theoptical fiber, at least one of an input EDFA connected in series withinput end of the optical fiber and an output EDFA connected in serieswith output end of the optical fiber; and a plurality of pumping lightsources each configured to provide pump light having a centralwavelength to the optical fiber via the WDM coupler, wherein the opticalfiber is configured to have a different dispersion characteristic thanat least one of the output optical transmission fiber and the inputoptical transmission fiber, and the central wavelength of each of thepumping light sources are different from each other and a wavelengthinterval between the plurality of pumping light sources or greater than6 nm and smaller than 35 nm.
 2. The optical repeater of claim 1,wherein: the optical fiber comprises at least one of a single modefiber, SMF, and a dispersion compensation fiber, DCF.
 3. The opticalrepeater of claim 2, wherein: the at least one of an input EDFA and anoutput EDFA comprises only one input EDFA; and the optical fiber isconnected to an output end of the only one input EDFA.
 4. The opticalrepeater of claim 1, wherein: at least one of the plurality of pumpinglight sources is configured to have an adjustable optical output levelthat produces an adjustable gain in the optical fiber; and the at leastone of an input EDFA and an output EDFA is configured to have anadjustable gain.
 5. The optical repeater of claim 1, wherein: theoptical fiber has a Raman wavelength gain dependency and the at leastone of an input EDFA and an output EDFA is configured have an EDFAwavelength gain dependency, wherein an optical output of the pluralityof pump light sources is set so that the Raman wavelength gaindependency configured to at least partially offset the EDFA wavelengthgain dependency.
 6. The optical repeater of claim 1, wherein: the atleast one of an input EDFA and an output EDFA comprises only one outputEDFA; and the optical fiber is connected to an input end of the only oneoutput EDFA.
 7. The optical repeater of claim 1, wherein: the at leastone of an input EDFA and an output EDFA comprises only one output EDFAand only one input EDFA, and the optical fiber is connected between theonly one output EDFA and the only one input EDFA.
 8. The opticalrepeater of claim 1, further comprising: a monitor signal detecting andcontrol signal generating circuit connected between the optical fiberand the plurality of pumping light sources, and configured to monitorand adjust levels of the pump light so as to control a gain applied tothe optical signal by the optical fiber.
 9. The optical repeater ofclaim 8, wherein: the monitor signal detecting and control signalgenerating circuit is configured to maintain a pre-determined constantdifference between an input level and an output level of a plurality ofWDM signals.
 10. The optical repeater of claim 9, wherein: the monitorsignal detecting and control signal generating circuit is connected tothe input end of the optical fiber.
 11. The optical repeater of claim 9,wherein: the monitor signal detecting and control signal generatingcircuit is connected to the output end of the optical fiber.
 12. Theoptical repeater of claim 11, wherein: the at least one of an input EDFAand an output EDFA consists of one of an input EDFA and an output EDFA,said one of an input EDFA and an output EDFA being configured to exhibitat least one of a constant optical amplifier gain control and a constantoptical amplifier output control; and the plurality of pumping lightsources is configured to be independently controlled by the monitorsignal detecting and control signal generating circuit to reduce aninter-channel optical repeater output signal deviation.
 13. The opticalrepeater of claim 1, further comprising: a second WDM coupler connectedto one of the input optical transmission fiber and the output opticaltransmission fiber and configured to receive a residual pumping lightfrom the plurality of pumping light sources.
 14. The optical repeater ofclaim 13, further comprising: a third WDM coupler connected to anotherone of the input optical transmission fiber and the output opticaltransmission fiber and configured to receive a residual pumping lightfrom the plurality of pumping light sources and to launch said residualpumping light into the another one of the input optical transmissionfiber and the output optical transmission fiber.
 15. The opticalrepeater of claim 1, further comprising: a second WDM coupler connectedto one of the at least one of the at least one of an input EDFA and anoutput EDFA and configured to receive a residual pumping light from theplurality of pumping light sources.
 16. The optical repeater of claim15, further comprising: a third WDM coupler connected to another one ofthe at least one of an input EDFA and an output EDFA and configured toreceive a residual pumping light from the plurality of pumping lightsources.
 17. The optical repeater of claim 1, wherein: said opticalfiber is configured to compensate for chromatic dispersion of the WDMoptical signal caused by the input optical transmission fiber.
 18. Theoptical repeater of claim 1, wherein: an amount of gain applied to theoptical signal resulting from the pump light from the plurality pumpinglight sources is set to compensate for an attenuation of the opticalsignal in at least one of the DCF and the input optical transmissionfiber.
 19. The optical repeater of claim 1, wherein: an amount of gainapplied to the optical signal resulting from the pump light from theplurality of pumping light sources is set to suppress an amplificationfluctuation of the EDFA and to maintain a repeater gain flatness.
 20. Amethod for optically amplifying a wavelength division multiplex opticalsignal in an optical fiber configured to be a Raman amplifying medium,comprising steps of: generating a plurality of pump lights; injectingthe plurality of pump lights to an optical fiber configured to be aRaman amplifying medium while the wavelength division multiplex opticalsignal propagates though the optical fiber; and controlling respectivelevels of the plurality of pump lights to adjust at least one of awavelength-dependent loss characteristic of the optical fiber and awavelength-dependent noise characteristic of the optical fiber.
 21. Themethod of claim 20, wherein: the optical fiber comprises at least one ofa single mode fiber and a dispersion compensating fiber, DCF, having apredetermined non-linearity characteristic; the plurality of pumpinglight sources comprise four semiconductor lasers with fiber Bragggratings, FBG, a polarization beam combiner, PBC, and a WDM coupler; andthe four semiconductor lasers are configured to generate pumping lightat different wavelengths.
 22. The method of claim 20, further comprisinga step of: combining the pumping lights via a Mach-Zehnderinterferometer wave combiner.
 23. The method of claim 22, furthercomprising steps of: selectively applying a subset of the pumping lightsto the amplification medium; and selectively setting a uniform inputpower to each of the subset pumping lights, wherein the subset of thepumping lights comprise a number of wavelengths from a short wavelengthside of a pumping light band greater than another number of wavelengthsat a long wavelength side of a pumping light band, and the step ofselectively applying a subset of the pumping lights is controlled toprovide a substantially uniform gain.
 24. A method for repeating awavelength division multiplex optical signal, comprising steps of:providing a pump light from a plurality of pumping light sources to anoptical fiber configured to be a Raman amplifying medium and to receivethe wavelength division multiplex optical signal from an opticaltransmission fiber; and amplifying the optical signal with at least oneof the plurality of pump light sources and at least one EDFA connectedin series with the optical fiber, wherein said amplifying step includeswavelength-dependent amplification.
 25. The method of claim 24, wherein:the optical fiber comprises at least one of a single mode fiber, SMF,and a dispersion compensation fiber.
 26. The method of claim 24,wherein: at least one of the plurality of pumping light sources and theat least one EDFA are configured to impart an adjustable gain on theoptical signal.
 27. The method of claim 26, further comprising a stepof: setting a gain of at least one of the plurality of pumping lightsources and the at least one EDFA so as to at least partially offset awavelength dependency of an EDFA gain.
 28. The method of claim 27,further comprising steps of: detecting a monitor signal; and controllinga signal generating circuit connected between the optical fiber and theplurality of pumping light so as to control a gain of the optical fiber.29. The method of claim 28, wherein: the controlling step is configuredto maintain a pre-determined difference between an input level and anoutput level of a plurality of WDM signals.
 30. The method of claim 24,further comprising steps of: controlling a gain of a first EDFA and asecond EDFA of the at least one EDFA to produce at least one of aconstant optical amplifier gain control and a constant optical amplifieroutput control; and independently controlling a gain of the plurality ofpumping light sources to reduce an inter-channel optical repeater outputsignal deviation.
 31. The method of claim 24, further comprising a stepof: pumping a residual pumping light to the input optical transmissionfiber through at least one of an input and an output side of the opticalfiber.
 32. The method of claim 24, further comprising: pumping aresidual pumping light input to the at least one EDFA.
 33. The method ofclaim 25, wherein: the plurality of pumping light sources is configuredto compensate for a loss of signal in at least one of the dispersioncompensation fiber and the optical transmission fiber.
 34. The method ofclaim 24, wherein: the plurality of pumping light sources is configuredto suppress a gain fluctuation of the EDFA and to maintain a repeatergain flatness.
 35. A Raman amplifying medium comprising: an opticalfiber configured to guide therein a WDM optical signal having a signalbandwidth of at least 20 nm, wherein said optical fiber is configured toreceive WDM pump light having a predetermined pump bandwidth and toproduce an amplification characteristic of a predetermined gain havingless than 1 dB of ripple across an amplification bandwidth that is atleast 20 nm and that overlaps said signal bandwidth, said pump lightcomprising light from a plurality of semiconductor lasers that producemultimode optical outputs having respective center wavelengths separatedfrom one another in an inclusive range of 6 nm through 35 nm.
 36. TheRaman amplifying medium of claim 35, wherein: the optical fibercomprises at least one of a single mode fiber and a dispersioncompensation fiber.
 37. The Raman amplifying medium of claim 35,wherein: the optical fiber comprises a dispersion compensation fiberhaving a dispersion property of less than −20 ps/nm per 1 km.
 38. TheRaman amplifying medium of claim 35, wherein: the optical fiber isconfigured to have a non linear index n2 of refraction equal to orgreater than 3.5×10⁻²⁰ m²/W.